1. Field of the Invention
This invention relates to a driveshaft housing for an outboard motor, and more particularly to an improved driveshaft housing for an outboard motor that is reinforced against expansion and contraction force exerted thereon.
2. Description of Related Art
A conventional outboard motor generally includes a power head, a driveshaft housing and a lower unit. The power head contains an internal combustion engine. The driveshaft housing depends from the power head and contains a driveshaft that is driven by said engine. In particular, a crankshaft of the engine drives the driveshaft. Since the crankshaft extends generally vertically in the power head, the driveshaft also extends generally vertically in the driveshaft housing. The lower unit depends from the driveshaft housing and contains a propulsion device such as a propeller. The driveshaft drives the propulsion device through a conventional transmission. Thus, the watercraft associated with the outboard motor is propelled by the outboard motor.
The engine usually has an exhaust system for discharging exhaust gases from its combustion chamber(s) to the body of water surrounding the outboard motor. The exhaust system has an exhaust conduit that extends through the driveshaft housing and the lower unit. Exhaust gases flow through the exhaust conduit downwardly and are finally discharged to the body of water through an opening, for example, formed in a propeller hub.
The exhaust gases, immediately after discharged from the engine into the exhaust conduit, have tremendous expansion pressure and this pressure acts on the wall of the driveshaft housing as well as on any internal walls within the driveshaft housing that define the exhaust conduit. The discharge of the exhaust gases intermittently and repeatedly occurs every exhaust stroke of the engine. Accordingly, the housing walls will be intermittently and repeatedly stressed by the fluctuating expansion pressures.
In the meantime, the driveshaft housing is configured generally as an oval shell in a plan view with its major axis extending between fore and aft ends of the housing. The lateral or side wall portions, therefore, are weaker (i.e., less rigid) than the fore and aft wall portions. If rigidity of these portions is not sufficient, vibration occurs when the aforenoted expansion pressure acts upon them. In addition, the engine per se generates relatively large vibration and this vibration is also transmitted to the housing shell. Hence, the lateral walls tend also to be stressed by these vibrations. If the frequency of the vibrations is consistent with the inherent frequency of vibration of the housing shell, resonance will occur and this results in discernable noise.